1. Field of the Invention
The present invention relates to a two-electrode welding method in gas-shielded arc welding.
2. Description of the Related Art
Techniques for a gas-shielded arc welding method have been proposed in International Publication No. WO2008/016084, Japanese Unexamined Patent Application Publication No. 2004-188428, and Japanese Unexamined Patent Application Publication No. 2010-125496. For example, International Publication No. WO2008/016084 makes an assumption, in tandem gas metal arc (GMA) welding, that there is a different shielding gas composition suitable for each of a leading electrode and a trailing electrode. This is because the leading electrode has a function of increasing the depth of weld penetration into a base material, whereas the trailing electrode has a function of stopping the flow of metal with arc pressure to shape a molten pool, the metal being melted by an arc from the leading electrode and flowing backward from the molten pool. Under the assumption described above, International Publication No. WO2008/016084 proposes a technique in which, by supplying shielding gases of different compositions suitable for the leading electrode and the trailing electrode, it is possible to stabilize the droplet transfer and reduce the amount of spatter without sacrificing the depth of weld penetration.
Specifically, International Publication No. WO2008/016084 proposes a technique in which a leading-electrode shielding gas to be supplied to the leading electrode is a two-component gas mixture of argon gas and carbon dioxide or a three-component gas mixture of argon gas, carbon dioxide, and oxygen gas; a trailing-electrode shielding gas to be supplied to the trailing electrode is argon gas alone, a two-component gas mixture of argon gas and carbon dioxide, a two-component gas mixture of argon gas and oxygen gas, or a three-component gas mixture of argon gas, carbon dioxide, and oxygen gas; and the concentration of carbon dioxide in the trailing-electrode shielding gas is lower than that in the leading-electrode shielding gas.
Japanese Unexamined Patent Application Publication No. 2004-188428 proposes a welding wire and a welding method using the welding wire to solve the problem with related art in which, in welding using a shielding gas containing at least 50% by volume of carbon dioxide, it is very difficult to achieve spray transfer. That is, Japanese Unexamined Patent Application Publication No. 2004-188428 proposes a welding wire and a welding method using the welding wire with which, in carbon dioxide gas-shielded arc welding using a shielding gas mainly composed of carbon dioxide, droplet spray transfer can be achieved and it is possible not only to reduce the amount of spatter but also to obtain a good bead shape even when high-speed welding is performed. Note that the shielding gas mainly composed of carbon dioxide refers to a shielding gas that contains, for example, at least 50% by volume of carbon dioxide.
Specifically, Japanese Unexamined Patent Application Publication No. 2004-188428 proposes a technique in which a rare-earth element is added to a welding wire and the content of calcium (Ca) is specified to stabilize and concentrate an arcing point in a negative electrode; a straight polarity, which is opposite the normal polarity, is used (i.e., the welding wire is negatively charged) to reduce the amount of spatter even when a larger amount of carbon dioxide is contained in the shielding gas; and titanium (Ti), zirconium (Zr), and aluminum (Al), which are strong deoxidizing elements, are added to the welding wire to achieve more stabilized weldability.
Japanese Unexamined Patent Application Publication No. 2010-125496 proposes a technique in which, in a tandem gas-shielded arc welding method of consumable electrode type which uses two welding wires as electrodes, an active gas containing at least 40% by volume of carbon dioxide (CO2) is used as a shielding gas for the leading electrode, an inert gas containing a total of at least 99.5% by volume of one or two of argon (Ar) gas, helium (He) gas, and hydrogen (H2) gas is used as a shielding gas for the trailing electrode, and slag is removed by the trailing electrode before molten metal produced in the leading electrode is solidified.
In the technique proposed in International Publication No. WO2008/016084, a gas mixture mainly composed of inert gas, such as argon gas, is used as a shielding gas to reduce the amount of spatter. However, this technique has a problem in that the weld penetration is as shallow as that in a tandem method of related art. Also, the technique proposed in International Publication No. WO2008/016084 has a problem in that the cost of the shielding gas is high. Since the cost of inert gas, such as argon gas, is several times higher than that of carbon dioxide, it has been desired, in actual welding practice, to reduce the use of inert gas and to use a gas mainly composed of carbon dioxide as much as possible.
The technique proposed in Japanese Unexamined Patent Application Publication No. 2004-188428 has a problem in that the rare earth element added to the welding wire is expensive. Also, although the technique proposed in Japanese Unexamined Patent Application Publication No. 2004-188428 is limited to a straight polarity, a large amount of spatter is produced if, for example, welding is performed with a straight polarity by using an inexpensive solid wire.
As with the technique proposed in Japanese Unexamined Patent Application Publication No. 2004-188428, the technique proposed in Japanese Unexamined Patent Application Publication No. 2010-125496 has a problem in that the rare earth element added to the welding wire is expensive. Additionally, the technique proposed in Japanese Unexamined Patent Application Publication No. 2010-125496 has a problem in that it is not possible to perform tandem gas-shielded arc welding if an active gas containing 100% by volume of CO2 is used as a shielding gas for the leading electrode and, at the same time, an inexpensive solid wire is used as the leading electrode. As a preferred embodiment of this technique, Japanese Unexamined Patent Application Publication No. 2010-125496 proposes a technique in which the leading electrode and the trailing electrode are made to have a straight polarity and a reverse polarity, respectively. This causes two arcs to repel each other and affects the process in which a weld bead is shaped by the trailing electrode, which is the greatest benefit of tandem gas-shielded arc welding.
Generally, welding which uses carbon dioxide as a shielding gas for the leading electrode has been known for providing deep weld penetration. However, due to a very large amount of spatter, it has been unable to use carbon dioxide in tandem gas-shielded arc welding.
The present invention has been made in view of the problems described above. An object of the present invention is to provide a tandem gas-shielded arc welding method in which, even though 100% carbon dioxide is used for the leading electrode, it is possible to achieve lower cost, less spatter, and deeper weld penetration than those in tandem gas-shielded arc welding of related art which uses a shielding gas mainly composed of inert gas.